Level II scour analysis for Bridge 71 (WODSTH00050071) on Town Highway 5, crossing Kedron Brook, Woodstock, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WODSTH00050071 on Town Highway 5 crossing Kedron Brook, Woodstock, Vermont
(figures 1–8). A Level II study is a basic engineering analysis of the site, including a
quantitative analysis of stream stability and scour (U.S. Department of Transportation,
1993). Results of a Level I scour investigation also are included in Appendix E of this
report. A Level I investigation provides a qualitative geomorphic characterization of the
study site. Information on the bridge, gleaned from Vermont Agency of Transportation
(VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is
found in Appendix D.
The site is in the New England Upland section of the New England physiographic province
in east-central Vermont. The 16.1-mi²
drainage area is in a predominantly rural and forested
basin. However, the bridge site is within the Village of Woodstock. In the vicinity of the
study site, the surface cover is best described as suburban downstream of the bridge and
forest and brush upstream of the bridge.
In the study area, Kedron Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 33 ft and an average bank height
of 11 ft. The predominant channel bed material is cobble with a median grain size (D₅₀) of
112 mm (0.368 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on September 14, 1994, indicated that the reach was vertically degraded. Evidence of
the degradation was observed at the outlet of the bridge where the stream bed is 4 ft below
the downstream invert of the structure (see figure 6).
The Town Highway 5 crossing of Kedron Brook is a 30-ft-long, two-lane bridge/box
culvert consisting of one 25-foot concrete span (Vermont Agency of Transportation, written
communication, August 3, 1994). The opening length of the structure parallel to the bridge
face is 23.5 ft.The bridge is supported by vertical, concrete abutments with wingwalls. The
channel bed under the bridge is covered entirely by a concrete slab. The channel is skewed
approximately 45 degrees to the opening and the opening-skew-to-roadway is also 45
degrees.
Scour countermeasures at the site include concrete retaining walls on both the left and right
downstream banks extending approximately 130 ft downstream; a drywall constructed of
stone on the upstream right bank extending to the next bridge upstream; type-2 stone fill
(less than 36 inches diameter) along the upstream left bank, at the upstream end of the
upstream right wingwall, and along the base of the retaining wall on the downstream left
bank; and type-3 stone-fill (less than 48 inches diameter) along the base of the retaining
wall on the downstream right bank. In addition, the channel under the bridge is concrete.
Further details describing conditions at the site are included in the Level II Summary and
Appendices D and E.
Scour depths and recommended rock rip-rap sizes were computed using the general
guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. Total
scour at a highway crossing is comprised of three components: 1) long-term streambed
degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow
area at a bridge) and; 3) local scour (caused by accelerated flow around piers and
abutments). Total scour is the sum of the three components. Equations are available to
compute depths for contraction and local scour and a summary of the results of these
computations follows.
Contraction scour for all modelled flows ranged from 0.0 to 2.5 ft. The worst-case
contraction scour occurred at the incipient roadway-overtopping discharge, which was less
than the 100-year discharge. The contraction scour depths do not take the concrete channel
bed under the bridge into account. Abutment scour ranged from 8.7 to 18.2 ft. The worstcase abutment scour occurred at the 500-year discharge. Additional information on scour
depths and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed elevations, based on the calculated scour depths, are presented in tables 1 and 2.
A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths
were calculated assuming an infinite depth of erosive material and a homogeneous particlesize distribution.
It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.